1. Field of the Invention
The present invention relates to a nitride semiconductor laser device having a ridge stripe structure, and a manufacturing method thereof.
2. Description of the Prior Art
Nitride semiconductor laser devices, where light oscillates in a range from an ultraviolet region to a visible region, are fabricated and manufactured as prototypes with the use of nitride semiconductor materials exemplified by GaN, AlN, InN and mixed crystals thereof. When such nitride semiconductor laser devices are manufactured, a pair of trenches which are parallel to each other are formed in the upper portion of a nitride semiconductor growth layer in a p-side electrode formation region, a ridge is formed at a portion between the pair of trenches and a p-side electrode is formed over the ridge and the pair of trenches in some types of nitride semiconductor lasers having a conventional ridge stripe-type waveguide structure. FIG. 9 shows a schematic cross sectional view of a nitride semiconductor laser device having such a structure.
The nitride semiconductor laser device of FIG. 9 has a configuration in that an undoped GaN buffer layer 2 grown at a low temperature, an undoped GaN layer 3, an n-type GaN contact layer 4, an n-type AlGaN clad layer 5, an n-type GaN optical waveguide layer 6, an InGaN multiple quantum well structure active layer 7, a p-type AlGaN cap layer 8, a p-type GaN optical waveguide layer 9, a p-type AlGaN clad layer 10, and a p-type GaN contact layer 11 are sequentially layered on a sapphire substrate 1 of which the surface is a C surface. The nitride semiconductor laser device includes trenches 12 and 13, a ridge 14, an insulating film 15, openings 15a and 15b, a p-side electrode 16 and an n-side electrode 17.
The upper layer portion of the n-type GaN contact layer 4, the n-type AlGaN clad layer 5, the n-type GaN optical waveguide layer 6, the InGaN multiple quantum well structure active layer 7, the p-type AlGaN cap layer 8, the p-type GaN optical waveguide layer 9, the p-type AlGaN clad layer 10 and the p-type GaN contact layer 11 form a mesa portion in mesa form having a predetermined width. In addition, the trenches 12 and 13 extending linearly in parallel to each other in the direction of a resonator are provided, for example, in the upper layer portion of the p-type AlGaN clad layer 10 and the p-type GaN contact layer 11 in this mesa portion, and the ridge 14 is formed between the trenches 12 and 13.
As shown in FIG. 9, the insulating film 15 made of SiO2 or the like, having a thickness of 300 nm is provided on the surface of the mesa portion and on the surface of the n-type GaN contact layer 4 in the portion other than the mesa portion. The opening 15a is provided in the portion of the insulating film 15 above the ridge 14, and the opening 15b is provided in the portion of the insulating film 15 above the n-type GaN contact layer 4 which is adjacent to the mesa portion. The p-side electrode 16 having a thickness of 410 nm is provided over the ridge 14 and the trenches 12 and 13. Thus, an ohmic contact is made between the p-side electrode 16 and the p-type GaN contact layer 11 of the ridge 14 through the opening 15a provided in the insulating film 15. In addition, the portion of the p-side electrode 16 above the ridge 14 and the portions of the p-side electrode 16 formed on the flat portions except for the trenches 12 and 13 are approximately at the same level in height. Furthermore, the n-side electrode 17 is formed in the opening 15b provided in the insulating film 15; thus, an ohmic contact is made with the n-type GaN contact layer 4.
The nitride semiconductor laser device manufactured in accordance with the aforementioned technique, however, is prone to the following problem. When wire bonding is carried out on the p-side electrode 16 using a wire made of gold or the like, in order to make an electrical connection to the outside, the joint portion between this wire and the p-side electrode 16 is damaged, and a leakage path of a current easily generates in the insulating film 15. This problem becomes more significant in nitride semiconductor laser devices made of gallium-based compounds. This problem will be described in detail by way of examples.
In order to confirm the aforementioned problem, a nitride semiconductor laser is fabricated in such a manner that the structure in which the opening 15a is not provided in the insulator 15 immediately above the ridge 14 has been modified from the nitride semiconductor laser device having the conventional structure shown in FIG. 9. As a result, no portion of the p-side electrode 16 makes direct electrical contact with the p-type GaN contact layer 11 in this nitride semiconductor laser device. FIG. 10 shows the current-voltage characteristics of the nitride semiconductor laser device when a voltage is applied thereto in the forward direction through the p-side electrode 16 and the n-side electrode 17 via gold wires joined to these electrodes in accordance with a ball bonding method where an end portion of a gold wire is fused by means of arc discharge into ball form, to which ultrasonic waves are applied so that the end portion of the gold wire is joined to an electrode. Here, the insulating film 15 is made of SiO2 and has a thickness of 350 nm. In addition, the p-side electrode 16 has a two layer structure where a Pd layer having a thickness of 15 nm and an Au layer having a thickness of 200 nm are sequentially layered in this order starting from the p-type GaN contact layer 11 side.
Although the entirety of the p-side electrode 16 is provided on the insulating film 15 in such a manner that no portion thereof makes direct electrical contact with the p-type GaN contact layer 11, as is clear from FIG. 10, a current starts flowing in the vicinity where the applied voltage value exceeds 3 V and a leak current of which the value is close to 10 mA flows when the applied voltage value is 6 V.
The aforementioned phenomenon strongly depends on the conditions when a gold wire is joined to the top of the p-side electrode 16 using a ball bonding method. That is, the period of time for applying ultrasonic waves is made long and the output of the ultrasonic waves is enhanced in order to strengthen the joint between the p-side electrode and the gold wire. When the period of time for applying ultrasonic waves is made long or the output of the ultrasonic waves is enhanced in this manner, however, a leak current tends to flow easily. In contrast, it has been found that in the case where the insulating film 15 is formed so as to have a thickness as large as, for example, approximately 800 nm or the p-side electrode 16 is formed so as to have a thickness as large as, for example, approximately 600 nm, a leak current can be suppressed.
If the insulating film 15 is formed so as to have a thickness of, for example, 800 nm in order to suppress the leak current in the aforementioned phenomenon, however, it becomes very difficult to provide the opening 15a. In the case where the opening 15a is formed in the insulating film 15 in accordance with a conventional method, a resist pattern having an opening is formed and the portion of the insulating film 15 of which the surface is exposed from this opening is removed using a wet etching method until the p-type GaN contact layer 11, which is the base, is exposed. In the case where the wet etching method is used, however, etching progresses isotropically in the lateral direction in addition to in the depth direction.
In such a case, etching progresses 800 nm in the depth direction and, also, etching progresses in the lateral direction, resulting in the progress of etching of 800 nm each on the right and left side, approximately 1.6 μm or more. However, the width of the ridge 14 is usually in a range from approximately 1.4 μm to 3.0 μm. Therefore, the possibility is high where the width of the opening 15a formed in accordance with a wet etching method becomes wider than the width of the ridge 14. Meanwhile, if a dry etching method is used, anisotropic etching becomes possible and etching in the lateral direction is prevented. However, at the same time, there is a risk that the p-type GaN contact layer 11 may be damaged by ion impact and the like.
In addition, also in the case where the p-side electrode 16 is formed so as to have a thickness as large as, for example, approximately 600 nm, influences of the distortion caused by the difference in the thermal expansion coefficients between the material of the p-side electrode 16 and the gallium nitride-based compound constituting the ridge 14 cannot be ignored; therefore, there is a risk that the InGaN multiple quantum well structure active layer 7 may be adversely affected.